It is well-known that people, such as the disabled, who are confined to a bed or a wheelchair for long periods of time frequently suffer from sores, which result from areas near the surface of the skin being deprived of blood due to pressure exerted on those areas by the bodily support under the person's body weight. In the context of beds it is known, for example from GB-A-1595417, to provide a mattress comprising an array of hollow flexible tubes which are sequentially inflated and deflated, so that in effect pressure ripples travel along the length of the mattress. Thus while the person's body is continuously supported by the mattress, the actual regions of the body in contact with the mattress which bear the body weight change with time. This prevents any one area of the body being continuously deprived of blood and thus discourages the formation of pressure sores.
It is known from EP-A-475593 to provide a cushion for the seat of a chair comprising at least two groups of inflatable tubes arranged in a side-by-side relationship and oriented in a direction corresponding to the front to back direction of the chair seat. The tubes of a first group are a single tube or two or more adjacent tubes and the tubes of the each or other group are two or more spaced apart tubes arranged on opposite sides of the first group of tubes. The tubes of each group are inflatable together by a control unit, so that the group of tubes which support the body weight of a person sitting in the chair alternate with time.
The change in the region of the user's body supported by the cushion when a group of tubes are deflated encourages "reactive hyperaemia". This is the beneficial increase in blood supply to the region of the user's body no longer supported by the cushion. Reactive hyperaemia is maximised by deflating the tubes quickly to a condition where the pressure at the interface between the deflated tubes and the user's body is low, for example between 5 and 20 mmHg.
In EP-A-475593 the cycle of inflation of the groups of tubes is as follows:
i) inflate first group of tubes to maximum inflation; PA1 ii) deflate second group of tubes; PA1 iii) inflate second group of tubes to maximum inflation; PA1 v) deflate first group tubes; PA1 vi) repeat from step (i).
However, it is found that this type of cushion suffers from the disadvantage that the control unit often needs to switch on to inflate the tubes due to small pressure fluctuations in them. This can cause discomfort to the person sitting on the cushion. It is a particular problem when the cushion is provided with a portable power supply, typically a battery in a wheelchair, as it leads to excessive drainage of the supply.
In addition, during the steps (ii) and (iv) in the cycle of inflation, the bulk of the weight of the person sitting on the cushion is carried by one group of tubes. This leads to a large increase in the pressure inside the inflated tubes and causes them to deform. This increases the pressure at the interface between the user and the deflated tubes, thereby reducing the benefit to the user from reactive hyperaemia.